Plants have evolved various defense mechanisms to resist infection by pathogens. Upon recognition, the host plant initiates one or more signal transduction pathways that activate various plant defenses and thereby avert pathogen colonization. In many cases, resistance is associated with increased expression of defense genes, including the pathogenesis-related (PR) genes and the accumulation of salicylic acid (SA) in the inoculated leaf. The SA signal transduction pathway plays a pivotal role in plant defense signaling (see Durrant and Dong, 2004). When SA accumulation is suppressed in tobacco (Nicotiana tabacum) and Arabidopsis (Arabidopsis thaliana) by expression of the nahG transgene, which encodes the SA-degrading enzyme SA hydroxylase, susceptibility to both compatible and incompatible pathogens is enhanced and PR gene expression is suppressed (Gaffney et al., 1993; Delaney et al., 1994). Similarly, Arabidopsis mutants that are impaired in SA responsiveness, such as npr1 (Cao et al., 1997; Ryals et al., 1997; Shah et al., 1997), or pathogen-induced SA accumulation, such as eds1 (Falk et al., 1999), eds5 (Nawrath et al., 2002), sid2 (Wildermuth et al., 2001), and pad4 (Jirage et al., 1999), exhibit enhanced susceptibility to pathogen infection and impaired PR gene expression.
In addition to the major phytohormone-mediated defense pathways, fatty acid (FA)-derived signaling has also started to emerge as one of the important defense pathways (Vijayan et al., 1998; Kachroo et al., 2001, 2003b, 2004; Weber, 2002; Li et al., 2003; Yaeno et al., 2004). Desaturation of stearic acid (18:0)-acyl carrier protein (ACP) to oleic acid (18:1)-ACP catalyzed by the SSI2/FAB2-encoded stearoyl-ACP desaturase (S-ACP-DES) is one of the key steps in the FA biosynthesis pathway that regulates levels of unsaturated FAs in the cell (FIG. 9). A mutation in ssi2 confers stunted phenotype, constitutive PR gene expression, spontaneous lesion formation, and enhanced resistance to both bacterial and oomycete pathogens (Kachroo et al., 2001; Shah et al., 2001). By contrast, the ssi2 plants are unable to induce jasmonic acid (JA)-responsive gene PDF1.2 and show enhanced susceptibility to necrotrophic pathogen Botrytis cinerea (Kachroo et al., 2001, 2003b). The activity of the mutant S-ACP-DES enzyme was reduced 10-fold, resulting in elevation of 18:0 content in ssi2 plants (Kachroo et al., 2001). However, an increase in 18:0 does not contribute to altered defense signaling because several ssi2 suppressors show wild type-like signaling and yet accumulate high levels of 18:0 (Kachroo et al., 2003a).
A mutation in ssi2 also results in reduction in 18:1 content. The altered morphology and defense phenotypes in the ssi2 plants are restored by a loss-of-function mutation in the ACT1-encoded glycerol-3-P (G3P) acyltransferase, or in the GLY1-encoded G3P dehydrogenase (G3PDH), both of which elevate 18:1 levels in the ssi2 plants (Kachroo et al., 2003b, 2004). A mutation in gly1 and act1 results in reduced carbon flux through the prokaryotic pathway, which leads to a reduction in the hexadecatrienoic (16:3) acid levels (Kunst et al., 1988, Miquel et al., 1998). However, the gly1 and act1 plants continue to show normal growth characteristics, suggesting that increased flux through the eukaryotic pathway compensates for their defect. Because both 18:1 and G3P are required for the acyltransferase-catalyzed reaction, a reduction in either is likely to reduce the carbon flux through ACT1.
The levels of G3P and 18:1 can also be modulated by exogenous application of glycerol. The glycerol treatment leads to an increase in the endogenous G3P levels, which results in quenching of 18:1. Since the ACT1-catalyzed step is rate limiting, the quenching of 18:1 is more drastic in glycerol-treated ACT1-overexpressing lines (Kachroo et al., 2004). A reduction in the 18:1 in wild-type plants confers phenotypes similar to that of the ssi2 mutant.
G3P, an obligatory component for the biosynthesis of all plant glycerolipids, is generated either via the G3P dehydrogenase (G3PDH)-catalyzed reduction of dihydroxyacetone phosphate (DHAP) or via the glycerol kinase (GK)-catalyzed phosphorylation of glycerol. Plants contain several cytosolic, mitochondrial and plastidial isoforms of G3PDH (Shen et al., 2003; Wei et al., 2001) and all these may to contribute to the total G3P pool. Low levels of plastidal G3P due to a mutation in the GLY1-encoded G3PDH has been shown to reduce the carbon flux through the prokaryotic pathway. A mutation in gly1 leads to a reduction in the hexadecatrienoic (16:3) acid levels and this phenotype can be complemented by exogenous application of glycerol (Miquel et al. 1998; Miquel. 2003). The gly1-1 plants show normal growth characteristics, suggesting that contributions from the other G3PDH isoforms and increased flux through the eukaryotic pathway compensates for the defect in gly1. The GLY1-encoded G3PDH has recently been shown to participate in defense signaling in Arabidopsis (Kachroo et al, 2004; Nandi et al, 2004).
In comparison to G3PDH, only one GK has thus far been identified in Arabidopsis. The GLI1 (previously NHO1)-encoded GK has been shown to be required for non-host resistance against bacterial isolates of Pseudomonas syringae and resistance against the necrotrophic pathogen, Botrytis cinerea (Kang et al, 2003). The reasons why a defect in GK affects resistance to both bacterial and fungal pathogens have not been fully identified and described. Since gli1 plants are impaired in glycerol catabolism they accumulate glycerol, and these levels peak shortly after germination (Eastmond, 2004). The high level of glycerol in gli1 plants may increase the tolerance of the gli1 seedlings to various abiotic stress treatments including salt, freezing, desiccation and hydrogen peroxide (Eastmond, 2004).
Glycerol plays a role in various metabolic processes, including its conversion to glycerol-3-phosphate (G3P), which serves as a building block for glycerolipid biosynthesis. In plants, G3P is synthesized via the glycerol kinase-mediated phosphorylation of glycerol or via the G3P dehydrogenase (G3PDH)-mediated reduction of dihyroxyacetone phosphate. Both GK and G3PDH participate in host-pathogen interactions (Kang et al., 2003; Nandi et al., 2004; Kachroo et al., 2004).
Pathogens differ from saprophytes not only in their ability to recognize and penetrate host tissues, but also in the capability to access the nutrients available there. The establishment and maintenance of a metabolic sink by the pathogen is a crucial aspect of pathogenesis, but it has received very little attention in comparison to signaling related to initial recognition, and we do not understand much about it (Asahi et al., 1979; Pennypacker, 2000; Solomon et al., 2003; Oliver and Ipcho, 2004). Biotrophy is widely believed to differ from necrotrophy in this process (Mendgen and Hahn, 2002; Schulz-Lefert and Panstruga, 2003; Oliver and Ipcho, 2004). However, the molecular intractability of obligate biotrophs has made it difficult to test this hypothesis rigorously.